1. Field of the Invention
The invention relates to optical waveguides including optical fibers. More particularly, the invention relates to writing Bragg gratings in optical fibers.
2. Description of the Related Art
Gratings are periodic changes in the index of refraction of the photosensitive core of an optical fiber. A region of periodic perturbations or modulations in the refractive index (e.g., a Bragg grating) is useful in causing reflection of a narrow range of wavelengths of the light transmitted in the fiber core, while allowing wavelengths outside the narrow range to pass through the fiber without reflection. Gratings of this kind are used, e.g., as filters.
The dimension of the grating period relative to the wavelength of the incident light and the length of the grating affect light transmission through the fiber in terms of wavelength range and percentage of light transmitted. Short period gratings, i.e., gratings having periodicities of approximately 1 .mu.m or less, are reflective gratings, which are useful for laser stabilization, sensing structural imperfections, and for adding and dropping channels in wavelength division multiplexed (WDM) systems. Long period gratings or transmissive gratings, i.e., those gratings having periodicities, e.g., within a range from approximately 50 .mu.m to approximately 1500 .mu.m, are useful as band-stop filters and gain-equalizers in optical amplifiers.
Conventionally, for a short-period grating, the refractive-index perturbations are written on the core of optical fibers by impinging a standing-wave interference pattern of ultraviolet (UV) or other suitable light along the length of the fiber using light launched from the fiber end (e.g., as disclosed in U.S. Pat. No. 4,474,427). In this technique, the periodicity of the grating is equal to that of the standing wave. Alternatively, gratings are written along the length of the fiber using an interference pattern created outside of the optical fiber by the angled application of several transverse beams of UV light (see e.g., U.S. Pat. No. 4,725,110 issued Feb. 16, 1988 to Glenn et al. and U.S. Pat. No. 4,807,950 issued Feb. 28, 1989 to Glenn et al.). In this manner, the grating periodicity is controlled by the angle of incidence of the interfering beams. Also, the resulting gratings reflected light at much longer wavelengths, e.g., at 1.55 .mu.m, a wavelength of interest for present-day optical fiber communication systems. Such techniques are especially suitable for writing short period gratings because of the feature accuracy that such techniques produce.
Further developments in writing short period Bragg gratings yielded the phase mask or phase-shifting mask. For example, see U.S. Pat. No. 5,367,588, issued Nov. 22, 1994 to Hill et al. A phase mask employs a surface relief pattern having grating striations shaped appropriately to modulate spatially the phase of the illuminating light, thus forming an appropriate interference pattern having the required periodicity. The interference pattern is imprinted (photoinduced) into the optical fiber via laser irradiation of the phase mask with UV light.
Techniques using phase masks that impinge interference patterns on a grating are highly accurate with respect to grating features and therefore are especially useful in the fabrication of short period gratings. However, such interference techniques typically are difficult to align, especially within a mass production-type environment. Furthermore, even the slightest degree of misalignment is magnified by the relatively high degree of accuracy of the grating features produced.
By comparison, long period gratings typically are characterized by longer periodicities and larger grating features and thus require less accuracy when writing. Therefore, grating writing techniques more conducive to the mass production of optical waveguide devices were developed as methods for writing long period gratings in optical fibers. One such method includes illuminating the fiber from the side through a mask having a slit therein (see, e.g., U.S. Pat. No. 5,104,209 issued Apr. 14, 1992 to Hill et al.). In this particular technique, the slit is equal in width to the desired width of the refractive index perturbations, which are written on the fiber in a point-by-point manner, with movement of the fiber with respect to the slit mask providing the periodicity of the grating region.
However, in forming gratings in optical fibers, the plurality of index perturbations written on the fiber core must have a consistent periodicity. Accordingly, a slit mask having a single slit therein is not well suited for the accuracy required in writing individual index perturbations with consistent periodicities. Moreover, the serial nature involved in writing the periodic index perturbations invites misalignment and reduced accuracy. Therefore, the slit mask evolved into the amplitude mask, which includes a plurality of periodic slits therein (see, e.g., U.S. Pat. No. 5,430,817 issued Jul. 4, 1995 to Vengsarkar). The amplitude mask's plurality of periodic variations in transmission is used to produce a grating along the core of an optical fiber. It is believed that amplitude masks have been demonstrated for transmissive, long period gratings only.
There is a need for fabricating reflective gratings in a manner that advantageously enjoys the beneficial aspects of both amplitude masks when used to form long period, transmissive gratings and of interferometric grating writing techniques when used to form short period Bragg reflectors. It is desired that such fabrication be more conducive to the alignment concerns of mass production techniques, yet without sacrificing the accuracy inherent in interferometric methods of writing gratings.